Allow demo'ing of any imager image

[Popolechien]

While we can demo staging ZIM files via https://clients.library.kiwix.org/, we can not do the same for individual hotspot images (much like we can for content bundles).

[rgaudin]

At the moment, it only supports auto-images (images exposed via the API) and is configured in operations

Without touching this much, we could add procedure/scripts to add arbitrary demo from a URL and subdomain that would be run by a tech on the demo server.

The additional limitation is of course the disk space on the demo server because setting up a demo needs twice the image's file size as empty space.